1. Field of the Invention
The present invention relates to a semiconductor device, and more particularly, to a method for forming an isolating layer in a semiconductor device.
2. Discussion of the Related Art
In general, during the fabrication of a semiconductor device, an active region having actual cells formed therein and a device isolating region for isolating the cells formed in the active region are formed on a semiconductor substrate. As the trend of high density semiconductor device packing continues, the securing of the active region with an accurate profile within the limited area of the substrate becomes increasingly important in semiconductor device fabrication. The most widely used technique in isolating the devices is the local oxidation of silicon (LOCOS) method.
A conventional method for forming a device isolating layer in a semiconductor device will now be explained with reference to the attached drawings. FIGS. 1a.about.1d are sectional views showing the steps of a LOCOS process.
Referring to FIG. 1a, a buffer oxide film 2 is formed on a semiconductor substrate (P-sub) 1, and a non-oxidizable nitride film 3 is formed on the buffer oxide film 2. As shown in FIG. 1b, a photoresist 4 is coated on the nitride film 3 and patterned to leave the photoresist 4 only on an active region, thereby defining the active region and the device isolating region. Then, the nitride film 3 and the buffer oxide film 2 are selectively removed using the patterned photoresist 4 as a mask. impurities (B.sup.+) having the same conductivity type as that of the semiconductor substrate 1 are injected into the device isolating region having the nitride film 3 and the buffer oxide film 2 removed therefrom. As shown in FIGS. 1c and 1d. a field oxide film is formed as a device isolating layer 5 in the device isolating region using the nitride film 3 as a mask. The nitride film 3 is then removed. During the formation of the device isolating layer 5, bird's beaks occur where the field oxide film 5 encroaches underneath the nitride film 3. As a result, the bird's beaks thus formed cause a non-uniform profile of the semiconductor substrate 1 in major and minor axis directions of the active region. For example, steps are formed along the major axis direction.
The steps formed during the formation of the device isolating layer 5 on the semiconductor substrate 1 will now be explained. FIG. 2 is a plan view showing directions of the bird's beak formation during the formation of the device isolating layer 5, and FIGS. 3a and 3b are sectional views of the semiconductor substrate 1 after an oxide film cleaning.
Referring to FIG. 2, during the formation of the field oxide film 5, a bird's beak effect occurs when the field oxide film 5 encroaches underneath the nitride film 3 in the active region. Here, when the width of the active region is not the same in all directions, i.e., the major and minor axis directions, the different widths of the mask, i.e., the nitride film 3, causes the non-uniform profile of the active region as shown in FIGS. 3a and 3b. The reason is that even though one bird's beak is formed in the minor axis direction, multiple bird's beaks are formed in the major axis direction. To compensate for this reduction of the active region by the abnormal growth of the field oxide film 5, the portion of the field oxide film 5 encroaching into the active region is removed by an oxide film cleaning. The oxide film cleaning is not a selective removal of the field oxide film 5, but an etch back of the field oxide film 5.
In the conventional process for forming a device isolating layer, expansions of the field oxide film in major and minor axis directions through the bird's beak effect are different. Therefore, in order to secure an active region, an oxide film cleaning (mostly in the major axis direction) is conducted to remove the portions of abnormally grown field oxide film. This causes partially different heights ("a" parts in FIG. 3b) in the active region along the major axis direction. In other words, where the oxide film is removed by the oxide cleaning, the height is reduced. The reduced height from the removal of the abnormally grown field oxide film portions, i.e., the portion which is lower than the active region, causes the degradation of N.sup.- --N.sup.-, N.sup.+ --N.sup.+, and P.sup.+ P.sup.+ junction space characteristics and the formation of humps. This in turn results in degradation of the device performance.